correct the version markers of errata additions by 775a3f0 and f9fe386
add chapter marker comments to errata
typo/grammar fix

the "that" in "more that two data" is almost certainly wrong. i think that "more than two data" might be right for some dialects of english, but a few lines beforehand (847), "pieces of data" is used, so i'm suggesting that. for consistency.
Merge pull request #410 from jkominek/patch-1

typo/grammar fix
Update induction.tex

missing space in Notes, line 2.
Update homotopy.tex

deleted duplicate `the'
Fix an overfull hbox in the proof of Theorem 7.2.2.
Merge pull request #421 from zhenlin/formatting

Fix an overfull hbox in the proof of Theorem 7.2.2.
Changed some inappropriate uses of \text... to \math…
Merge pull request #422 from zhenlin/formatting

Some inappropriate uses of \text... to \math…
fixed a typo in basics